Mail merge error when a data source is accessed by multiple users in Word

Symptoms

Consider the following scenario:

• You try to open a mail-merged document in Microsoft Word 2010, Word 2013, or Word 2016.
• The data for this document is a Microsoft Excel worksheet (*.xlsx) that's stored in shared folder.
• The document has already been opened.

In this scenario, you receive the following error message:

Error has occurred: External table is not in the expected format.

Cause

When you store an .xlsx file in a shared folder and use it as data source in a Word mail merge, Word opens the .xlsx file exclusively. If another user tries to open a Word file that uses the same data source simultaneously, only read-only permissions can be granted, and therefore the request fails.

Workaround

To work around this issue, use a .csv or .xls file instead of an .xlsx file as the data source. For example, open your data source in Excel, and then save it as an Excel 97-2003 Workbook (*.XLS) or CSV Comma Delimited (*.CSV) file.